The present invention relates to a process for producing an artificial leather. More particularly, it relates to a process for efficiently producing an artificial leather having an expanded polyurethane layer including fine foams, which has an excellent touch, appearance and a full-hand similar to that of natural leather and further has an excellent softness and an excellent surface strength.
Artificial leather is usually produced from various synthetic resins, such as urethane resins, amino acid resins, acrylic resins and nylon resins, by a dry process or a wet process. However, conventional artificial leather is inferior to natural leather in physical properties and touch. Moreover, the conventional process has some defects, such as a difficulty in the control of the conditions in the production steps. Also, certain environmental pollution problems are produced because harmful solvents are used.
It has been proposed to produce artificial leather from an expanded polyurethane produced by contacting an urethane prepolymer having a terminal isocyanate group (hereinafter, referred to as "urethane prepolymer") with moisture and an organic substance having an activated hydrogen, because the expanded polyurethane layer has a similar touch to that of the natural leather. However, it is generally very difficult to prepare a composition comprising as the essential component a urethane prepolymer. And, even if the composition can be prepared, when an artificial leather is produced from the composition, the leather has an inferior surface strength, and further, it is technically difficult to produce the optimum thick touch to the artificial leather. Thus, nobody has succeeded in the production of an artificial leather having excellent touch, full-hand, etc., similar to that of natural leather by using a urethane prepolymer.
The present inventors have now found a process for producing an artificial leather similar to natural leather by using a urethane prepolymer, i.e. by preparing a composition containing the urethane prepolymer wherein an appropriate catalyst, foam stabilizer, colorant, filler or the like are incorporated (no solvent is used), coating the composition thus prepared onto a release paper, laminating a base sheet onto the coating layer, and then foaming and resinifying the composition at a constant foaming ratio (Japanese Patent laid Open publication (without examination) No. 35301/1975 and Japanese Patent Application No. 95813/1974). The artificial leather produced by this process has excellent properties, but the process has still some defects in the stability and efficiency of the production.
As the results of the present inventors' further extensive studies, it has been found that the desired artificial leather similar to the natural leather can be constantly and efficiently produced by using a combination of an amine catalyst and a metallic catalyst and carrying out the process by two stage reactions, i.e. by proceeding previously with the foaming reaction, laminating the base sheet thereon, providing a pressure until the foaming layer achieves a fixed thickness and then heating the resultant composite, whereby the aging step necessary in the conventional process can be omitted.
An object of the present invention is provide a process for producing an artificial leather having an expanded polyurethane layer including fine foams and having excellent touch, appearance and full-hand similar to those of natural leather and having excellent softness and surface strength.
Another object of the present invention is to provide an improved process for producing an artificial leather by using a combination of an amine catalyst and a metallic catalyst and controlling the reaction conditions so that the expansion ratio of the urethane prepolymer is maintained in a fixed range.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
According to the present invention, the artificial leather is produced as follows:
Firstly, a composition comprising as the essential component an urethane prepolymer is prepared by admixing a catalyst, a foam stabilizer, and optionally other additives, to the urethane prepolymer which is liquid at room temperature and is prepared from a polyol and an organic polyisocyanate in a ratio of the isocyanate group equivalent [NCO] to the hydroxy group equivalent [OH] of 2.0 to 4.0, and then admixing thereto an inert gas so that plenty of extremely fine cells (foams) are included in the compound and agitating the mixture. The composition thus prepared is then coated onto a release paper, or onto a layer of a surface treating agent coated on the release paper, in a thickness of 0.03 to 0.5 mm, and the resultant is expanded in an atmosphere of a temperature of 40.degree. to 95.degree. C (dry-bulb thermometer) and a humidity of 60% or more (relative humidity) so that the number of the foams (fine cells) in the coating (foaming) layer becomes 5,000 forms/inch.sup.2 or more and the density of the coating layer becomes 80 - 700 g/liter and is simultaneously resinified so that the coating layer becomes semi-solid, the surface of which has enough tack. Thereafter, onto the coating layer is laminated a base sheet under a pressure so that the thickness of the coating layer becomes about 0.95 to 0.4 times that before the laminating of the base sheet, and the resultant product is sufficiently reacted by heating at a temperature of 90.degree. to 180.degree. C., and thereafter, the release paper is peeled off. When the composition is directly coated on the release paper (i.e. not on the layer of the surface treating agent coated thereon), the leather obtained above may optionally be treated with a surface treating agent.
According to a suitable embodiment of the present invention, the composition comprises, as the essential component thereof, the urethane prepolymer, wherein an inert gas is incorporated, which is coated onto a release paper or a layer of a surface treating agent coated thereon with an appropriate device, such as a doctor knife or a roll coater. The resultant is passed through steam at a fixed temperature and humidity (hereinafter, refered to as "steam treatment"), and a base sheet is piled on the coating layer and the resultant is pressed, for instance, by passing between two rollers having a fixed space or between two rollers to which a fixed pressure is given, and then heated at a fixed temperature (hereinafter, referred to as "heat treatment"). Thereafter, the release paper is peeled off to give an artificial leather having a composite structure comprising the expanded polyurethane layer and the base sheet, which is optionally treated with a surface treating agent. The artificial leather thus obtained has excellent touch, appearance and full-hand similar to that of natural leather and further has excellent softness and surface strength.
The composition comprising as the essential component an urethane prepolymer is prepared by adding a catalyst and a foam stabilizer, and optionally a colorant, filler and other additives, to a liquid urethane prepolymer, which is prepared from a polyol and an organic polyisocyanate in a ratio of the isocyanate group equivalent [NCO] to the hydroxy group equivalent [OH] of 2.0 to 4.0, kneading the mixture and then incorporating an inert gas into the resulting composition which has preferably a viscosity of 70 to 1,000 poise.
The polyol includes polyester polyols and polyether polyols and a mixture thereof, which are liquid at room temperature, said polyester polyols and polyether polyols containing 2 to 4 functional groups (OH group) and having an equivalent weight of 300 to 50,000. Suitable examples of the polyols are as follows:
(i) Liquid polyester polyols obtained by copolyesterifying three or more polycondensable components, i.e. liquid multicomponent polyester polyols produced from (a) two or more polyols, such as straight diols having 2 to 12 carbon atoms (e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, or 1,6-hexanediol), ether glycols (e.g. diethylene glycol, or triethylene glycol), branched diols having 3 to 12 carbon atoms (e.g. 1,2-propanediol, 1,3-butanediol, neopentyl glycol, or hexylene glycol), and triols having 3 to 12 carbon atoms (e.g. trimethylolethane, trimethylolpropene, or glycerin), and (b) two or more polycarboxylic acids, such as aliphatic dicarboxylic acids (e.g. adipic acid, succinic acid, glutalic acid, pimelic acid, suberic acid, azelaic acid, or sebacic acid), aromatic polycarboxylic acids (e.g. phthalic acid, or trimellitic acid). Suitable combinations of the polyols and the polycarboxylic acids are the combination of butanediol, diethylene glycol and adipic acid; ethylene glycol, 1,4-butanediol and adipic acid; and ethylene glycol, trimethylolpropane, adipic acid and phthalic acid.
(ii) Liquid polyol mixtures comprising a polyester polyol, which is produced from two polycondensable components and is not or almost not liquid, and a polyester polyol, which is produced from another combination of polycondensable components, or a liquid polyester polyol, for instance, a mixture of (a) an aliphatic polyester diol which is solid at room temperature (e.g. polyethylene glycol adipate, polytetramethylene glycol adipate, or polyhexamethylene glycol adipate), (b) a liquid two component polyester polyol, such as a polyester diol obtained from a branched glycol and a dicarboxylic acid (e.g. 1,2-propanediol adipate, 1,3-butanediol adipate, or neopentyl glycol adipate), or a polyester polyol obtained from an ether glycol and a dicarboxylic acid (e.g. diethylene glycol adipate), and/or (c) a liquid polyether polyol. Suitable examples are a polyol mixture of polyethylene glycol adipate and diethylene glycol adipate; and a polyol mixture of a polytetramethylene glycol adipate and a polypropylene glycol.
(iii) Liquid polyol mixtures comprising a monocomponent polyester polyol obtained from a hydroxycarboxylic acid or a lactone thereof and a liquid polyol, for instance, a mixture of (a) a monocomponent polyester polyol obtained from 6-hydroxycaproic acid or .epsilon.-caprolactone and (b) the liquid polyol mixture of a polyester polyol or polyether polyol as mentioned in the above (ii). Suitable examples are a polyol mixture of a poly(.epsilon.-caprolactone) glycol and a polyether polyol of a glycerin-propylene oxide adduct.
(iv) Liquid polyether polyols, such as polyether polyols (e.g. polytetramethylene glycol, polypropylene glycol, glycerin-propylene oxide adduct, or trimethylolpropane-propylene oxide adduct), chipped polyether polyols wherein an ethylene oxide is added to the terminal, or polyether polyols graft-polymerized with acrylonitrile, which are used alone or in a combination thereof.
The organic isocyanate may be the conventional compounds which are usually used for the preparation of urethane prepolymers. Suitable examples are tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate and hexamethylene diisocyanate.
When at least one polyol as mentioned above is reacted with at least one organic polyisocyanate, there can be obtained a liquid urethane prepolymer having an excellent workability. When the urethane prepolymer is prepared by using a polyol mixture, the same urethane prepolymer may be alternatively prepared by preparing separately two urethane prepolymers by using each polyol alone and then mixing together the urethane prepolymers thus obtained.
The catalyst is used for the purpose of promoting the expansion of the urethane prepolymer during the steam treatment and further of promoting the crosslinkage and resinification thereof during the heat treatment. It has been unexpectedly found that it is very important to use a specific combination of certain catalysts for producing the desired artificial leather. Suitable combination of the catalysts comprises an amine catalyst (e.g. N,N,N',N'-tetramethylhexamethylenediamine, N,N-dimethylcyclohexylamine, N,N-dimethylhexylamine, tetramethyl-1,3-butanediamine, N,N,N',N',N'-pentamethyldiethylene-triamine, N,N-dimethylbenzylamine, triethylenediamine, triethylamine, and/or N-methylmorpholine) and a metallic catalyst, such as tin, cobalt, iron, zinc and lead containing catalysts, preferably, tin catalysts, such as dibutyl tin laurate, stannous octate, Marukure UL-1 (a trade name of a tin catalyst, made by Argus Co.). The amine catalyst and the metallic catalyst are used in an amount of 0.01 to 10 parts by weight, preferably 0.02 to 5 parts by weight, and 0.05 to 10 parts by weight, preferably 0.1 to 5 parts by weight, respectively, to 100 parts by weight of the urethane prepolymer. By using the specific combination of the catalysts, the reaction is divided into two stages: the steam treatment and the heat treatment, and thereby the desired artificial leather having constant properties can be efficiently produced.
The foam stabilizer may be commercially available silicon compounds (e.g. siloxane-oxyalkylene copolymers which are on market in the trade name of SH-192 and SH-194 (by Toray Silicon K.K.) and L-540, L-544, L-532 and L-5350 (by Nippon Unicar K.K.) and is used in an amount of 0.1 to 5 parts by weight to 100 parts by weight of the urethane prepolymer.
In the process of the present invention, an inert gas is incorporated into the composition, by which plenty of extremely fine foams (cells) are included in the composition and further the foaming layer formed by the subsequent steam treatment has an extremely fine foam structure. In the so-called "one-shot process", the size of the foams in the foaming layer can be controlled by changing the ring bore or the shape of the outlet of the agitating and mixing vessel, controlling the reaction of the polyol and the isocyanate, or using an appropriate foam stabilizer. However, in the process, wherein a composition is coated on a release paper and the coating layer is expanded by the steam treatment, as in the present invention, it is very difficult to control the size of the foams by an external means, and it is necessary to have a plenty of extremely fine cells of an inert gas included within the compound. The inert gas includes nitrogen gas, fluorohydrocarbon gases (e.g. Freon: a trade name of the product made by Du Pont), dried air, or the like. The inert gas is included in the composition in the form of extremely fine cells having an average diameter of not more than 1/10 mm, preferably from 1/50 mm to 1/100 mm. Thus, by incorporating plenty of the extremely fine cells of the inert gas into the composition, which shows a turbid appearance, the foaming layer having the desired density and size of the forms can be formed by the subsequent steam treatment, and thereby the desired artificial leather having similar properties to those of the natural leather can be produced.
The composition including plenty of extremely fine cells of the inert gas is then coated onto a release paper or a layer of the surface treating agent coated on a release paper in a thickness of 0.03 to 0.5 mm, preferably 0.05 to 0.4 mm. The thickness of the coating layer within this range is very important for carrying out uniformly the expansion reaction in the subsequent steam treatment and further for giving the foaming layer having uniform properties.
In the present invention, the reaction is divided into two stages, i.e. the steam treatment and the heat treatment.
In the first stage, the composition coated onto a release paper is subjected to the steam treatment prior to the laminating of the base sheet, and thereby the foaming and the resinification of the coating layer proceed well. The steam treatment is carried out at a temperature of 40.degree. to 95.degree. C. and at a humidity of 60% or more, preferably 80 to 95%, until the coating layer becomes semi-solid by resinification thereof, so that the number of the foams in the coating layer becomes 5,000 forms/inch.sup.2 or more, preferably 10,000 forms/inch.sup.2 or more and the density of the coating layer becomes 80 - 700 g/liter. The time for the steam treatment may vary with the amount of the isocyanate contained in the urethane prepolymer, the kind and amount of the catalysts, and the conditions of the steam treatment, but may be usually from 15 seconds to 2 minutes, by which the semi-solid foaming layer having a tacky surface is formed.
In the second stage, a base sheet is laminated onto the coating layer thus treated, and the resultant is subjected to the heat treatment.
The base sheet is not limited to a specific sheet, but may be, for instance, the commercially available, raised or normal, woven or knitted fabrics, non-woven fabrics, or these fabrics penetrated with a synthetic resin. The laminating is done under a pressure so that the thickness of the coating layer becomes 0.95 to 0.4 time of that before the laminating. The control of the laminating conditions is effective for the surface strength of the product, the adhesion strength between the coating layer and the base sheet, the soft hand of the product, and further the occurrence of small puckering. The laminating is usually performed by passing between two rollers having a fixed space or between two rollers to which a fixed pressure is given, but may be done by any other method.
After the laminating of the base sheet, the coating layer is further subjected to the heat treatment at a temperature of 90.degree. to 180.degree. C., by which the foaming layer is completely resinified. According to this process, it is not required to subject to the aging step for a long time which is required for completing the reaction in the conventional process. The time for the heat treatment may vary with the amount of the isocayanate contained in the urethane prepolymer, the kind and amount of the catalysts, the degree of the steam treatment and the conditions of the heat treatment, but may be usually from about 1 to about 7 minutes.
In the present invention, the use of the release paper coated with a surface treating agent or the treatment of the product with a surface treating agent can give the artificial leather having more excellent color shade, brightness and touch. The surface treating agent include conventional urethane resins, amino acid resins, polyamide resins, polycarbonate resins, nitrocellulose, acrylic resin, which may be used alone or in a combination thereof.